The present invention relates generally to the field of electronics, and, more specifically, to a charge transformer used to enhance the performance of low capacitance electrometers in detector applications, such as single electron transistors.
The discovery of the Coulomb blockade phenomena, which arises from the discreteness of electric charge, and the relatively recent advances in nanofabrication techniques, have led to the ability to make new kinds of devices that allow one to control and measure the motion of a single electron, hereinafter referred to as xe2x80x9ccharge sensitive devices.xe2x80x9d One known example of such a device is a single electron transistor (SET), which is an extremely precise solid-state electrometer. Indeed, a single electron turns an SET on or off. SETs have been used in metrological applications, as well as a tool for imaging localized individual charges in semiconductors. Further, it is likely that SETs will be used in connection with several different types of new applications, ranging from astronomy to quantum computer read-out circuitry. SETs are advantageous in many applications because of their superior charge sensitivity relative to typical commercial electrometers.
However, despite the many orders of magnitude better charge sensitivity of SETs relative to commercial electrometers, it has not been advantageous to use SETs as an electrometer when the device under test has a large relative capacitance because of excessive noise. As a result, SETs have heretofore not been used in connection with commercial macroscopic devices.
To illustrate the problem more specifically, FIG. 1 illustrates a SET amplifier circuit, which is constructed to detect an electrical current generated by a device under test 10. The device under test 10 is functionally represented in FIG. 1 as the combination of a current source ISig and a capacitor CD. The device under test 10 is connected to the SET such that the charge generated by the device under test determines the current flow through the SET, as known by one of the ordinary skill in the art. Thus, the charge of the device under test can be monitored via the potential at the output terminal 12. This SET circuit can be viewed as an electrometer for the device under test. Note, however, the noise performance of this circuit is far from optimum when the capacitance of the device under test is much larger than that of the SET.
Specifically, it is known that the capacitance of an SET (denoted as Cset) is constrained by the operation temperature of the SET. Therefore, assuming the SET is operated at approximately room temperature, the capacitance of the SET, Cset, is limited to a very low value, typically less than 1 aF. In contrast, the capacitance (CD) of the device under test 10 is likely to be much larger than the Cset, particularly for commercial macroscopic devices. As a result, only an unacceptably small fraction of the input power from the device under test 10, given by Cset/CD, can couple to the SET. Thus, when the capacitance CD of the device under test 10 is in the picoFarrad (pF) or nanofarrad (nF) range, the suppression factor becomes too large. As a result, charge sensitive devices, such as SET amplifiers, heretofore have not been used to measure and detect macroscopic devices.
Therefore, the inventors hereof have recognized the need for a new device for and method of implementing charge sensitive devices, such as SET amplifiers, to detect and measure electrons generated by macroscopic devices.
The present invention relates to a charge transformer having a plurality of capacitors and a plurality of switches connected between the capacitors, wherein the switches are adapted to alternatively connect the capacitors in series and in parallel. When the switches are in a first state, the capacitors are connected in parallel, and the capacitors are capable of being charged. When the switches are in a second state, the capacitors are connected in series, and the capacitors are capable of being discharged.
In certain embodiments of the invention, a charge transformer is connected between a device under test and a charge sensitive device, such as an SET. The charge transformer facilitates the coupling of charge from the device under test to the charge sensitive device. In this way, a significant portion of the power generated by the device under test is coupled to the charge sensitive device.
One of ordinary skill in the art will recognize various other advantages and configurations of the present invention.